A fuel injection system for an internal combustion engine includes a common rail and accumulator injectors.
A diesel engine is a compression ignition engine. That is to say, the engine includes a cylinder in which a piston compresses air to raise its temperature, and fuel is injected into the cylinder where it mixes with the compressed, heated air, ignites and burns, releasing energy to drive the engine. A fuel injection system operates cooperatively with the engine to pressurize the fuel and to inject it into the cylinder as a mist or cloud of small droplets. An accumulator injector as may be used in such a fuel injection system receives pressurized fuel and includes a chamber controlled by a two-way valve in which the pressurized fuel accumulates until released by a needle valve through a nozzle. The needle valve is controlled by opposing forces exerted by the pressurized fuel. At a particular time during engine operation, one of the forces is relieved when the fuel exerting it is diverted (“spilled”) through a spill port, permitting the needle valve to open, whereupon the injector injects a charge of pressurized fuel into an engine cylinder.
The pressurized fuel accumulated in the chamber of the accumulator injector is injected in a very short pulse wherein the rate of injection is initially very high and falls rapidly to the end of injection. A particularly desirable feature of the pulse of fuel when injected through a nozzle is formation of an expanding cloud of fuel droplets that burn quickly and cleanly. In this regard, in conventional fuel injection systems, the injection begins when the pressure in the injector is sufficiently high enough to cause an injection valve to open. Since the injector is usually directly connected to an injector pump, the pressure in the injector increases during the injection cycle until cutoff occurs. The pressure rise causes the velocity of the injected stream of fuel to increase during the injection period with the result that the earlier portions of the injected stream, that have been slowed by the high density of compressed combustion air, are overtaken by the higher velocity of the later-injected stream, and agglomeration of the fuel droplets occurs. Such large droplets are then poorly evaporated and incompletely burned, resulting in the formation of soot and CO. In an accumulator injector, the pressure profile is reversed, with the later portions of the injected fuel stream having a lower velocity than the initial portions. The result is a desirable expanding cloud of fuel droplets characterized by absence of agglomeration.
An accumulator injector is typically provided as an integral electromechanical unit that includes an accumulator volume, a two-way valve, a needle valve assembly, a nozzle, a spill port and a solenoid mechanism to control the operation of the injector by actuating spilling through the spill port. Such a construction results in a relatively elongate injector assembly that complicates engine layout. Furthermore, if engine design requires more than one injector per cylinder, parametric variations and uneven heating may require the addition of control circuitry to synchronize solenoid responses of the multiple injectors.